At present, improvements to secondary batteries such as lithium-ion secondary battery are being actively made, since it is possible to achieve a high voltage and a high energy density with a secondary battery. Main components of a secondary battery are an assembly of a power generating element body composed of a pair of electrodes, namely, a positive electrode and a negative electrode, and a separator which separates both electrodes to prevent a short circuit therebetween, an electrolyte filled in the power generating element body, and an outer can which stores these components therein.
A lithium secondary battery that has been put to practical use is manufactured through the following procedure. Both positive and negative electrodes and a separator are inserted into an outer can body in a state of being wound in an overlap manner or being laminated on each other, and an opening of the outer can body is closed with a cap. Thereafter, an electrolyte is injected through an injection hole provided in the cap, and then the injection hole is sealed.
When the electrolyte is filled into the outer can, the electrolyte has to infiltrate into the entirety of the electrode assembly obtained by assembling the positive and negative electrodes and the separator. However, each of the gaps between the positive and negative electrodes and the separator in the electrode assembly which is a wound body or laminated body of the positive and negative electrodes and the separator which are in the form of sheet is very narrow, and it takes time until air having entered the gaps is replaced with the electrolyte that has newly infiltrated into the gaps and the electrolyte completely permeates and infiltrates into the gaps. Thus, it is necessary to inject the electrolyte, to store the outer can that remains open in a wide clean room that is adjusted into an optimum environment by its humidity and temperature being managed, for causing the electrolyte to gradually infiltrate into the gaps in the electrode assembly, and to cause the outer can to stand still until the replacement is completed. This method has a problem that it takes an excessive amount of time to cause the electrolyte to permeate into the electrode assembly and the electrolyte cannot be efficiently filled therein.
Thus, Patent Literature 1 has been proposed as improvement of the electrolyte filling method. A method described in Patent Literature 1 is a method in which the pressure in an outer can in which an electrode assembly is stored is reduced and an electrolyte is supplied under the reduced pressure, and the electrolyte is sucked by the pressure-reduced outer can as in a dropper, and injected and filled therein. In this method, the electrolyte is sucked into the outer can that is kept at a negative pressure, and thus the injection filling time is significantly shortened as compared to the conventional replacement method. However, micro air bubbles remaining in the gaps in the electrode assembly are increased in volume while the pressure is reduced, and a state is kept in which the air bubbles are stuck in the gaps. Thus, the electrolyte does not reach this portion, and a state is provided in which small bores are present. Accordingly, this method still is problematic for a demand for dense filling of the electrolyte.
Furthermore, as a solution to the problem of this method, a method described in Patent Literature 2 has been proposed. In this method, after the inside of an outer can is made into a negative pressure by suction, the electrolyte is injected. In a state of the electrolyte being injected, pressure is applied to decrease the volumes of micro air bubbles remaining stuck in the gaps, such that it is made easy for the air bubbles to float up from the gaps, whereby the degree of filling is increased. The pressure reduction and the pressure application are repeated. By this method, the injection rate is further increased. However, the buoyancy of the micro air bubbles is merely used, and there are micro air bubbles that do not float up due to the surface tension thereof or the like. Thus, the demand for dense filling of the electrolyte cannot be fully met. Other than the above, a filling method with only pressure application (Patent Literature 3) and a method using a centrifugal force (Patent Literature 4) have also been proposed.